FIG. 1 of the accompanying drawings depicts a well 2 containing water from an inlet 3. The water is shown filling the well to a level 4. Rungs 10 are provided within the well 2 in order to facilitate human service calls. In order to prevent overfilling of the well 2, lead and lag pumps 6 and 7 are provided to pump out water via an outlet 8. The lead and lag pumps 6 and 7 are operated by a controller 12 which monitors level 4 of the water.
One approach to monitoring the water level 4 in the past has been to make use of a probe 14 bearing multiple electrodes 16. As the water level rises, a low impedance, which controller 12 is able to monitor, occurs between the electrodes 16 on the probe 14 which are in contact with the water. Such a probe is described in U.S. Pat. No. 4,739,786 by the present inventor.
A problem identified by the current Inventor with water level sensing probe 14 when used in sewerage wells, is that the one or more electrodes may become covered with fat or other impurities 18. This is particularly the case about the duty or lead point of the well, i.e. the range that the level 4 assumes most of the time under normal conditions. Impurities 18 may cause shorting between electrodes 16 above or below the actual level of the water so that controller 12 is unable to accurately control lead and/or lag pumps 6 and 7 to maintain a desired fluid level. For example, if the lead pump 6 is activated before the correct level is reached, then the controller 12 will determine that the volume of water in the well has suddenly increased steeply and will use this incorrect figure to calculate pump down volume.
Similarly, if a very conductive liquid, such as salt water ingresses into the pit in beach side areas, premature activation of the pumps or alarms will occur. Alternately, if contaminates, such as oils, enter the pit, sensors become insulated and fail to detect the liquid level which can result in a total failure of the conductivity probe and subsequent flooding of the pit.
One approach to addressing the problem described above that has been attempted in the past is to install a pressure sensor 20 in order to provide controller 12 with an additional indication of water level. However, it is very difficult to physically and reliably install a separate pressure transducer 20 at an exact distance lower than the lowest electrode of the probe 14. This is particularly so because the pump cut-out level, where pressure sensors are usually intended to be installed as a backstop, can be over 10 meters below the well opening. Workplace health and safety regulations usually prohibit workers from climbing into pit 2, especially down to such a depth, to ensure that both probe 14 and pressure sensor 20 are at the correct level.
In any case, both the probe 14 and the pressure sensor 20 need to be removed for cleaning at regular intervals. The removal and reinstallation result in variations of the distance between the pressure sensor 20 and the bottom electrode and also of the calibration of the pressure sensor.
If installation of probe 12 and sensor 20 is attempted without entry into well 2 then it is virtually impossible to see and locate them accurately due to ongoing inflow into the well and associated spray.
Sewerage wells should be completely pumped out from time to time in order to avoid the build-up of excessive fat in the well. If the pumps are not turned off once the low water level is reached then the pumps may pump dry. However, variations in sensitivity of the pressure sensor may prevent the controller from accurately detecting the low water level cut out point. This results in either dry pumping, if the level is determined to be higher than it actually is, or less than optimal pumping out if the level is determined to be lower than it actually is.
To provide reliable fluid level calibration, the exact distance from the pressure transducer module 20 to each electrode must be known and needs to remain exactly the same throughout the life of the probe 14. If this distance changes, perhaps due to stretching and movements of the probe 14, then all calibration rules and formulas have to be updated resulting in much work and inaccuracies.
International patent application PCT/AU2006/000844 (WIPO publication No. WO2006/133511), to the present applicant, describes electrode fluid level sensing probe that terminates with a pressure sensor. Such a probe overcomes the problem of accurately locating the pressure sensor at a predetermined distance from the lowermost electrode. However, to date pump controllers have not been devised to properly take advantage of this new type of water level sensing probe.
It is an object of the present invention to provide a method and associated pump controller which offer improvements over those hitherto known in the art and which can process data from a probe of the type described in WO2006/133511.